Compositional and structural gradients for fuel cell electrode materials

ABSTRACT

A fuel cell includes at least one electrode operatively disposed in the fuel cell, and having a catalytically active surface. The present invention further includes a mechanism for maintaining a substantially uniform maximum catalytic activity over the surface of the electrode.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to fuel cells, and moreparticularly to fuel cells having electrodes with compositional and/orstructural gradients.

[0002] Fuel cells use an electrochemical energy conversion of a fuel andoxidant into electricity and heat. It is anticipated that fuel cells maybe able to replace primary and secondary batteries as a portable powersupply. In fuel cells, the fuel (containing a source of hydrogen orother oxidizable compound) is oxidized with a source of oxygen toproduce (primarily) water and carbon dioxide. The oxidation reaction atthe anode, which liberates electrons, in combination with the reductionreaction at the cathode, which consumes electrons, results in a usefulelectrical voltage and current through the load.

[0003] As such, fuel cells provide a direct current (DC) voltage thatmay be used to power motors, lights, electrical appliances, etc. A solidoxide fuel cell (SOFC) is one type of fuel cell that may be useful inportable applications, as well as in many other applications.

[0004] A significant amount of effort has been expended in optimizingcomposition and porosity of electrodes. Typical approaches have involvedelectrodes formed from materials having a constant compositional andstructural morphology. More recently, a structural and/or compositionalgradient of the electrode in the direction away from the electrolyteappears to provide some benefit in improving performance of SOFCsystems. Unfortunately, in both cases, compromises are necessarily maderelating to operating temperatures, fuel cell performance, and fuelutilization when using materials with such morphologies.

SUMMARY OF THE INVENTION

[0005] The present invention solves the drawbacks enumerated above byproviding a fuel cell including at least one electrode operativelydisposed in the fuel cell, and having a catalytically active surface.The present invention further includes a mechanism for maintaining asubstantially uniform maximum catalytic activity over the surface of theelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Objects, features and advantages of embodiments of the presentinvention may become apparent upon reference to the following detaileddescription and drawings, in which:

[0007]FIG. 1 is a semi-schematic cross-sectional perspective view of anembodiment of the present invention, showing fuel cell assemblies andthe gas flow passage in an embodiment of a single chamber fuel cell;

[0008]FIG. 1A is a schematic view an alternate embodiment of a singlechamber fuel cell;

[0009]FIG. 1B is a schematic view of another alternate embodiment of asingle chamber fuel cell;

[0010]FIG. 1C is a schematic view of an embodiment of a dual chamberfuel cell, showing in phantom optional inlet(s) downstream for fueland/or air;

[0011]FIG. 2 is a block diagram of an embodiment of an anode fuel/airmixture gradient for a single chamber fuel cell, schematically showing amanifold and the flow passage in phantom;

[0012]FIG. 3 is a block diagram showing an embodiment of an anodecompositional gradient;

[0013]FIG. 4 is a block diagram showing a further embodiment of an anodecompositional gradient;

[0014]FIG. 5 is a block diagram showing an embodiment of an anodestructural gradient;

[0015]FIG. 6 is a block diagram of an embodiment of a cathodecompositional gradient;

[0016]FIG. 7 is a block diagram of an embodiment of a cathode structuralgradient;

[0017]FIG. 8A is a block diagram of an embodiment of an anode portion ofa fuel cell stack;

[0018]FIG. 8B is a block diagram of an embodiment of a cathode portionof a fuel cell stack; and

[0019]FIG. 9 is a schematic representation of an embodiment of a methodfor making an anode having compositional gradient(s).

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The present invention is predicated upon the unexpected andfortuitous discovery that performance of a fuel cell may be improved byvarying the composition and/or structure of fuel cell electrodes(anodes/cathodes) with respect to the distance from the gas inlet tomaximize the catalytic activity to specific reactions related to thecomposition of the gas along the flow path, and/or by varying thecomposition of the gas (fuel and/or oxidant) over the catalyticallyactive surface of an electrode (with or without compositional and/orstructural gradients).

[0021] It is to be understood that, throughout this disclosure, thedefinition of “structure” and/or “structural” is intended to includemorphology, porosity, crystalline structure, and the like.

[0022] For anodes, the fuel near the inlet is predominantly ahydrocarbon, but along the flow path, reforming or partial oxidationprocesses can produce carbon monoxide and hydrogen, which may be amongthe dominant gases further downstream, especially under conditions ofhigh fuel utilization. Although catalysts that can reform, oxidize, orpartially oxide a hydrocarbon fuel can typically oxidize carbonmonoxide, hydrogen, and/or other partial oxidation products, they arenot optimized for these gases. In contrast, embodiments of the presentinvention choose compositional and/or structural gradients of the anodematerial so as to increase catalytic activity of the anode dependingupon where in the flow path the anode or discrete area of the anode ispositioned.

[0023] With regard to cathodes, the air near the inlet has not yet beendepleted of oxidants (e.g. is rich with oxygen), whereas furtherdownstream, the air becomes partially depleted or substantially depletedof oxidants. Embodiments of the present invention choose compositionaland/or structural gradients of the cathode material so as to increasecatalytic activity of the cathode depending upon where in the flow paththe cathode or discrete area of the cathode is positioned.

[0024] Referring now to FIG. 1, an embodiment of the fuel cell of thepresent invention is designated generally as 10. Fuel cell 10 includes aflow passage 24 having a gas stream flowing therethrough in thedirection of arrow A. Fuel cell 10 further includes at least oneelectrode 16, 18 operatively disposed in the flow passage 24. Theelectrode may be an anode 16 and/or a cathode 18. The electrode(s) arepart of a fuel cell assembly 12, which includes an electrolyte 14, ananode 16 disposed on one side of the electrolyte 14, and a cathode 18disposed on the same or the other side of the electrolyte 14. It isgenerally desirable for fuel cell 10 to include a plurality of fuel cellassemblies 12.

[0025] It is to be understood that the fuel cell 10 may be one of solidoxide fuel cells, proton conducting ceramic fuel cells, alkaline fuelcells, Polymer Electrolyte Membrane (PEM) fuel cells, molten carbonatefuel cells, solid acid fuel cells, and Direct Methanol PEM fuel cells.In an embodiment of the present invention, fuel cell 10 is a solid oxidefuel cell.

[0026] In the embodiment of FIG. 1, the fuel cell 10 is an example of asingle chamber fuel cell. In single chamber fuel cells, it may bedesirable to space apart adjacent fuel cell assemblies 12 so as topromote gas transport to more of the catalytically active surfaces ofthe anode 16 and cathode 18. The stacking order of these cells may beanode 16/gas channel 46/anode 16; or anode 16/gas channel 46/cathode 18.Alternatively and optionally, the cell 10 could be stacked anode16/electrolyte 14/cathode 18/electrolyte 14/anode 16/electrolyte 14,etc. without gas channels 46 between adjacent fuel cell assemblies 12.

[0027] The electrode 16, 18 has at least one discrete, catalyticallyactive area, the composition and/or the structure of which ispredetermined based upon an expected composition of the gas stream towhich the discrete area is exposed. If the electrode is anode 16, thediscrete catalytically active areas are designated as 16′, 16″ and 16′″.Although three discrete areas 16′, 16″ and 16′″ are shown, it is to beunderstood that anode 16 may include any number of discretecatalytically active areas as desired, or may continuously vary alongthe indicated direction. If the electrode is cathode 18, the discretecatalytically active areas are designated as 18′, 18″ and 18′″. As withanode 16, it is to be understood that although three discrete areas 18′,18″ and 18′″ are shown, cathode 18 may include any number of discretecatalytically active areas as desired, or may continuously vary alongthe indicated direction.

[0028] Fuel cell 10 further includes an inlet 20 adjacent an entrance toflow passage 24, wherein the electrode 16, 18 has an inlet end region 26proximate the inlet 20, and wherein the discrete area 16′, 18′ islocated at the inlet end region 26. It is to be understood that inlet 20may be an inlet for fuel, oxidants, or both fuel and oxidants. If theelectrode is an anode 16, the expected composition of the gas stream atthe inlet end region 26 is generally substantially unreformedhydrocarbon fuel. As such, according to an embodiment of the presentinvention, the composition and/or the structure of discrete area 16′ isoptimized for substantially unreformed hydrocarbon fuel.

[0029] If the electrode is a cathode 18, the expected composition of thegas stream at inlet end region 26 is a gas stream substantiallyundepleted of oxidants. As such, according to an embodiment of thepresent invention, the composition and/or the structure of discrete area18′ is optimized for a gas stream substantially undepleted of oxidants.

[0030] Fuel cell 10 further includes an outlet 22 adjacent an exit fromflow passage 24. The electrodes 16, 18 have an outlet end region 28proximate the outlet 22. Discrete areas 16′″, 18′″ are located at theoutlet end region 28.

[0031] If the electrode is an anode 16, the expected composition of thegas stream at outlet end region 28 is at least one of substantiallyreformed or partially reformed hydrocarbon fuel, byproducts thereof, andmixtures thereof. As such, according to an embodiment of the presentinvention, the composition and/or the structure of discrete area 16′″ isoptimized for at least one of substantially reformed or partiallyreformed hydrocarbon fuel, byproducts thereof, and mixtures thereof.

[0032] If the electrode is a cathode 18, the expected composition of thegas stream at outlet end region 28 is a gas stream substantiallydepleted of oxidants. As such, according to an embodiment of the presentinvention, the composition and/or structure of discrete area 18′″ isoptimized for a gas stream substantially depleted of oxidants.

[0033] Flow passage 24 has a midpoint 30, and the electrode 16, 18 has amidpoint region 32 proximate midpoint 30. Discrete area 16″, 18″ islocated at the midpoint region 32.

[0034] If the electrode is an anode 16, the expected composition of thegas stream at midpoint region 32 is at least one of substantiallyunreformed or partially reformed hydrocarbon fuel, byproducts thereof,and mixtures thereof. As such, according to an embodiment of the presentinvention, the composition and/or structure of discrete area 16″ isoptimized for at least one of substantially unreformed or partiallyreformed hydrocarbon fuel, byproducts thereof, and mixtures thereof.

[0035] If the electrode is a cathode 18, the expected composition of thegas stream at midpoint region 32 is a gas stream partially depleted ofoxidants. As such, according to an embodiment of the present invention,the composition and/or structure of discrete area 18″ is optimized for agas stream partially depleted of oxidants.

[0036] An electronic device according to the present invention includesan electrical load L, and fuel cell 10 connected to the load L. Anembodiment of a method of using fuel cell 10 includes the step ofoperatively connecting the fuel cell 10 to electrical load L and/or toan electrical storage device S. The electrical load L may include manydevices, including, but not limited to any or all of computers, portableelectronic appliances (e.g. portable digital assistants (PDAs), portablepower tools, etc.), and communication devices, portable or otherwise,both consumer and military. The electrical storage device S may include,as non-limitative examples, any or all of capacitors, batteries, andpower conditioning devices. Some exemplary power conditioning devicesinclude uninterruptible power supplies, DC/AC converters, DC voltageconverters, voltage regulators, current limiters, etc.

[0037] It is also contemplated that the fuel cell 10 of the presentinvention may, in some instances, be suitable for use in thetransportation industry, e.g. to power automobiles, and in the utilitiesindustry, e.g. within power plants.

[0038] Alternate embodiments of single chamber fuel cells are shown inFIGS. 1A and 1B.

[0039] Referring now to FIG. 1C, an embodiment of a dual chamber fuelcell is shown, whereby air (as a source of oxidant) is fed to thecathode 18 side, and fuel (as a source of reactant) is fed to the anode16 side. An optional additional air inlet 42 is shown in phantomdownstream from inlet 20; and an optional additional fuel inlet 44 isshown in phantom downstream from inlet 20. It is to be understood that,although only one additional air/fuel inlet 42, 44 is shown, there maybe any number of inlets 42, 44 as desired. Further, there may beadditional air inlet(s) 42 with or without additional fuel inlet(s) 44,and vice versa. It is to be further understood that a manifold and/orsimilar apparatus, operatively and fluidly connected to the anode 16side of flow passage 24 and/or to the cathode 18 side of flow passage24, may be provided for adding oxidants and/or fuel in at least one areadownstream from inlet 20. This may aid in ensuring efficient reactionsdownstream from the inlet 20, given the composition of the air/fuel andthe particular electrode material at a given location. By adding extraair (i.e. oxygen) at different locations on the cathode 18 side,downstream from the inlet 20, partial and total oxidation of the fuelcan be controlled. This may reduce the temperature gradient, increasethe fuel utilization and improve the performance of the fuel cell 10.Coking may be controlled by reducing the concentration of the fuel atspecific locations on the anode 16, and furthermore, may reduce thetemperature gradient on the anode 16. Coking may be defined as theconversion of small chain hydrocarbons to an inactive layer of carboncompounds that modify the catalyst in such a way as to reduceperformance.

[0040] Referring now to FIG. 2, fuel cell 10 is a single chamber fuelcell (FIGS. 3-9 may relate to either single or dual chamber fuel cells)and may further optionally include additional inlets and/or a manifold34 (shown schematically in FIG. 2) and/or similar apparatus, operativelyand fluidly connected to flow passage 24, for adding oxidants, fueland/or a fuel/air mixture in at least one area downstream from inlet 20.This may aid in ensuring efficient reactions downstream from the inlet20, given the composition of the fuel and the particular electrodematerial at a given location. By adding extra air (i.e. oxygen) atdifferent locations downstream from the inlet 20, partial and totaloxidation of the fuel can be controlled. This may reduce the temperaturegradient, increase the fuel utilization and improve the performance ofthe fuel cell 10. By adding extra fuel and/or an air/fuel mixture atdifferent locations downstream from the inlet 20, the dilution effect(due to hydrocarbon fuels' production of reaction product(s)) may becontrolled.

[0041] Along the fuel path, the fuel may react to form water, carbondioxide, carbon monoxide and H₂. Exhaust will result in a dilutioneffect, and air adds N₂ as well. Conventional fuel cells have a singleratio of fuel to air along the reaction path, whereas in embodiments ofthe present invention, the ratio of fuel to air is varied along thereaction path. In addition, a single chamber design of a fuel cell 10according to an embodiment of the present invention may have acompositional gradient of both the anode 16/cathode 18 material, and thegas phase reactants (adding air downstream to control the composition ofthe gas).

[0042] Referring now to FIG. 3, in a non-limitative embodiment of theanode 16 of the present invention, discrete area 16′ has as a maincomponent thereof. LaCr(Ni)O₃, the composition of discrete area 16″ hasas a main component thereof. La(Sr)CrO₃, and the composition of discretearea 16′″ has as a main component thereof. La(Sr)Cr(Mn)O₃. This is anexample of a compositional gradient of the anode material 16 whichallows for more complete utilization of the fuel. Higher performance maybe obtained by controlling the catalyst and the resulting gascomposition. The LaCrO₃ perovskite system of FIG. 3 is onenon-limitative example of optimizing catalytic activity of an anode 16according to embodiments of the present invention.

[0043] Doping the A and B sites of the perovskite lattice maysignificantly alter the observed catalytic activity and selectivity. Thenomenclature is A(C)B(D)O₃, where A and B are the specific sites in theperovskite structure, and C and D are the dopants on the sites.

[0044] It has been observed that LaCr(Ni)O₃ is good for methaneconversion and reforming reactions, La(Sr)CrO₃ is good for carbonmonoxide oxidation, and La(Sr)Cr(Mn)O₃ is good for hydrogen oxidation.

[0045] It is to be understood that material systems other than thosedescribed herein may be used as well, depending on the desiredcharacteristics and the fuels to be used.

[0046] Referring now to FIG. 4, in an alternate non-limitativeembodiment of the anode 16 of the present invention, the composition ofdiscrete area 16′″ includes a first amount of nickel in an anodematerial (for example, a samaria doped ceria (SDC)), the composition ofdiscrete area 16″ includes a second amount of nickel, which is less thanthe first amount of nickel, and the composition of discrete area 16′includes a third amount of nickel, which is less than the second amountof nickel.

[0047] Nickel assists in reaction of hydrocarbons. However, nickel(and/or other metals which assist in reaction of hydrocarbons) may causeundesirable temperature gradients which may lead to cracking of the fuelcell 10. For example, with Ni—SDC, most of the reaction occurs proximatethe fuel inlet, and this causes a temperature gradient (the fuel cellfilms/film stacks are hotter at the inlet 20 than at the outlet 22). Anembodiment of the present invention as shown in FIG. 4 provides a moreconstant temperature across the anode 16/fuel cell 10 by lowering theamount of nickel at the fuel inlet 20. This compositional gradient ofthe anode material 16 allows for more even heating of the anode 16during exothermic reactions so that the inlet end region 26 of the anode16 does not become overheated. This may reduce stress related todifferent thermal expansion at different regions of the fuel cell 10.

[0048] With better control over the heat given off by the exothermicreactions, other components of the fuel cell 10 may advantageously beoptimized for the lower temperature operation.

[0049] In addition or alternatively to selectively varying the ratio ofnickel and/or other metals, it is contemplated as being within thepurview of the present invention to vary the ceramic ratio, vary doping,etc.

[0050] Referring now to FIG. 5, a non-limitative embodiment of an anode16 structural gradient is shown. The structure of discrete area 16′″includes pores 36, the structure of discrete area 16″ includes pores 36smaller than the pores 36 in discrete area 16′″, and the structure ofdiscrete area 16′ includes pores 36 smaller than the pores 36 indiscrete area 16″. Structural gradients, in porosity, as well as threephase boundary length, may be controlled at different regions of theanode 16 of embodiments of the present invention. More porous anodes 16may be used at regions with higher exhaust compositions to reducediffusion limitations in the transport of reactive species to theelectrocatalytically active areas; for example, larger pore sizes at theportion 16′″ of the anode 16 reduce diffusional losses related to thetransport of fuel (with a high concentration of CO₂ and H₂O present) tothe three phase boundary in the anode 16. It is to be understood thatFIG. 5 is a very simplified representation. For example, the structuralgradient may not simply be smaller to bigger pores 36, it may be anode16 with different pore size distribution(s), e.g. dual distributionwhich may be a combination of large, transport pores for fasterdiffusion and nanopores with a higher concentration of catalyticcenters.

[0051] Referring now to FIG. 6, a non-limitative embodiment of a cathode18 compositional gradient is shown. It is to be understood that themain/base material for cathode 18 may be any suitable material, forexample, it may be chosen from examples of cathode materials listedbelow. In an embodiment, an example of a suitable main material forcathode 18 is Sm(Sr)CoO₃ (SSCO).

[0052] The composition of discrete area 18′″ includes a first amount ofmaterial catalytically more active (than the main/base cathode 18material) for the electrochemical reduction of molecular oxygen. Themore catalytically active material may aid the reduction of oxygen indepleted atmospheres. It is to be understood that this morecatalytically active material may be any suitable material. In anembodiment, this more catalytically active material is at least one ofplatinum, ruthenium, rhodium, silver, mixtures thereof, and the like.

[0053] The composition of discrete area 18″ includes a second amount ofthe more catalytically active material which is less than the firstamount of the more catalytically active material, and further includes afirst amount of material catalytically less active than the main/basecathode 18 material. It is to be understood that this catalytically lessactive material may be any suitable material. In an embodiment, thiscatalytically less active material is at least one of iron, manganese,mixtures thereof, and the like.

[0054] The composition of discrete area 18′ includes a second amount ofthe catalytically less active material, which is more than the firstamount of catalytically less active material. Without being bound to anytheory, it is believed that the addition of the less catalyticallyactive material will typically result in less active materials than thepure main material (e.g. SSCO), but may better match the thermalexpansion properties of the other components in the fuel cell 10. Sincethe inlet usually runs hotter, this may help reduce delamination orother stress in the cell.

[0055] Referring now to FIG. 7, an embodiment of a cathode 18 structuralgradient according to the present invention is shown. The structure ofdiscrete area 18′″ includes pores 38; the structure of discrete area 18″includes pores 38, which are smaller than the pores 38 in discrete area18′″; and the structure of discrete area 18′ includes pores 38 smallerthan the pores 38 in discrete area 18″. It is believed that increasingthe size of pores 38 downstream from inlet 20 advantageously allowshigher diffusional mass transport to the active areas in the cathode 18when there is a low(er) concentration of molecular oxygen in the airstream. It is to be understood that FIG. 7 is a very simplifiedrepresentation. For example, the structural gradient may not simply besmaller to bigger pores 38, it may be cathode 18 with different poresize distribution(s), e.g. dual distribution which may be a combinationof large, transport pores for faster diffusion and nanopores with ahigher concentration of catalytic centers.

[0056] According to embodiments of the present invention, thecompositional and/or structural gradient for the electrodes may also beincorporated into fuel cells stacks. The composition and/or structure ofa specific anode 16/cathode 18 in the stack may be predeterminedrelative to its position along the gas flow path. Referring now to FIGS.8A and 8B, embodiments of fuel cell stacks 40, 40′ are shown inschematic block diagrams. It is to be understood that when anode(s) 16is shown (as in FIG. 8A), it is (although not shown) associated with anadjacent electrolyte 14 and cathode 18 to form a fuel cell assembly 12.Likewise, when cathode(s) 18 is shown (as in FIG. 8B), it is to beunderstood that it is associated with an adjacent electrolyte 14 andanode 16 to form a fuel cell assembly 12.

[0057] Fuel cell stack 40, 40′ includes an inlet 20, an outlet 22, and aflow passage 24 disposed between inlet 20 and outlet 22 and having a gasstream flowing therethrough. A plurality of electrodes 16, 18 isoperatively positioned within the flow passage 24 from proximate inlet20 to proximate outlet 22 and positions therebetween. According to anembodiment of the present invention, the structure and/or thecomposition of each of the plurality of electrodes 16, 18 ispredetermined based upon an expected composition of the gas stream at anarea of the fuel cell stack 40, 40′ in which the electrode ispositioned.

[0058] In FIG. 8A, each of the plurality of electrodes is an anode 16;and in FIG. 8B, each of the plurality of electrodes is a cathode 18.Although three anodes/cathodes A, B, C are shown, it is to be understoodthat fuel cell stacks 40, 40′ may include any number of individualanodes 16/cathodes 18 as desired and/or necessitated by a particular enduse. As non-limitative examples, anode/cathode A, anode/cathode B andanode/cathode C are as follows.

[0059] According to an embodiment of the present invention, thecomposition and/or the structure of anode A is optimized forsubstantially unreformed hydrocarbon fuel. According to an embodiment ofthe present invention, the composition and/or the structure of cathode Ais optimized for a gas stream substantially undepleted of oxidants.

[0060] According to an embodiment of the present invention, thecomposition and/or the structure of anode C is optimized for at leastone of substantially reformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof. According to an embodiment ofthe present invention, the composition and/or structure of cathode C isoptimized for a gas stream substantially depleted of oxidants.

[0061] According to an embodiment of the present invention, thecomposition and/or structure of anode B is optimized for at least one ofsubstantially unreformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof. According to an embodiment ofthe present invention, the composition and/or structure of cathode B isoptimized for a gas stream partially depleted of oxidants.

[0062] It is to be understood that the compositions and/or structures ofanode/cathode A, B, C may be chosen from any examples given above. As anon-limitative example, the composition of anode A may have as a maincomponent thereof. LaCr(Ni)O₃, which is an example given above fordiscrete area 16′. The composition of cathode A may be SSCO with anamount of iron (which is an example given above for discrete area 18′)larger than an amount of iron in cathode B. Similarly, any of theexamples, and/or combinations thereof, of compositions and/or structuresgiven for discrete areas 16′/18′, 16″/18″ and 16′″, 18′″ may be used foranode/cathode A, anode/cathode B and anode/cathode C, respectively.

[0063] Further, it is to be understood that in addition to thecomposition and/or structure of each anode/cathode A, B, C beingindividually uniform (as described immediately hereinabove), any, someor all of the individual anodes/cathodes A, B, C may have compositionaland/or structural gradients thereon (e.g. anode A may include any, allor further discrete areas 16′, 16″ and 16′″).

[0064] Referring now to FIG. 9, one method of growing anode materialswith compositional gradients is shown. An embodiment of a method formaking a fuel cell anode 16 includes the step of depositing a first filmon a first end region 16′ of a substrate, wherein the first film ispreferentially catalytically active toward substantially unreformedhydrocarbon fuel. The method may further include the step of depositinga second film on a second end region 16′″ of the substrate opposed tothe first end region 16′, wherein the second film is preferentiallycatalytically active toward at least one of substantially reformed orpartially reformed hydrocarbon fuel, byproducts thereof, and mixturesthereof.

[0065] The method may further optionally include the step of depositingan intermediate film on a region 16″ of the substrate intermediate thefirst end region 16′ and the second end region 16′″, wherein theintermediate film is preferentially catalytically active toward at leastone of substantially unreformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof.

[0066] It is to be understood that the dashed lines between discreteareas 16′, 16″ and 16′″ in FIG. 9 represent that this embodiment of themethod of the present invention generally results in a gradientdistribution of the deposited materials, i.e. a continuous,inhomogeneous distribution. It is to be further understood that, in anyof the embodiments discussed herein, discrete areas 16′, 16″, 16′″, 18′,18″, 18′″ may also have a gradient distribution between adjacent areas,i.e. a continuous, inhomogeneous distribution.

[0067] In an embodiment of the method of the present invention, each ofthe first, intermediate, and second films has as a main componentthereof a nickel-samaria doped ceria cermet, and the method furtherincludes the step of biasing inclusion of nickel toward the second film.

[0068] In an alternate embodiment of the method of the presentinvention, the first film has as a main component thereof. LaCr(Ni)O₃,the intermediate film has as a main component thereof. La(Sr)CrO₃, andthe second film has as a main component thereof. La(Sr)Cr(Mn)O₃.

[0069] An embodiment of a method of the present invention for making afuel cell cathode 18 includes the step of depositing a first film on afirst end region 18′ of a substrate, wherein the first film ispreferentially catalytically active toward a gas stream substantiallyundepleted of oxidants. The method may further include the step ofdepositing a second film on a second end region 18′″ of the substrateopposed to the first end region 18′, wherein the second film ispreferentially catalytically active toward a gas stream substantiallydepleted of oxidants.

[0070] The method may further optionally include the step of depositingan intermediate film on a region 18″ of the substrate intermediate thefirst end region 18′ and the second end region 18′″, wherein theintermediate film is preferentially catalytically active toward a gasstream partially depleted of oxidants.

[0071] In an embodiment of the method of the present invention, each ofthe intermediate and second films has therein an amount of a materialcatalytically more active (than a main/base cathode 18material/substrate) for the electrochemical reduction of molecularoxygen (some suitable, non-limitative examples of the more catalyticallyactive material are as described hereinabove). The method furtherincludes the step of biasing inclusion of the more catalytically activematerial (e.g. platinum) toward the second film.

[0072] In an embodiment of the method of the present invention, each ofthe first and intermediate films has therein an amount of a materialcatalytically less active than a main/base cathode 18 material/substrate(some suitable, non-limitative examples of the less catalytically activematerial are as described hereinabove). The method further includes thestep of biasing inclusion of the less catalytically active material(e.g. iron) toward the first film.

[0073] Without being bound to any theory, it is believed thatembodiments of the method of the present invention may result in changesin morphology/structure, as well as in composition. Angular depositionmay result in porous materials depending on many factors, two of whichfactors are adatom mobility (material and temperature dependent, alsodependent on other parameters which may affect the energy of the adatomwhen it reaches the surface of the substrate: process pressure, power,substrate bias, target-to-substrate distance, and the like), andself-shadowing due to nucleation and growth of islands (due to the lowdeposition angles).

[0074] As such, it is to be understood that the first, intermediate andsecond anode films may also include pores 36 (such as pores 36 indiscrete areas 16′, 16″ and 16′″, respectively, as shown in FIG. 5),and/or other changes in morphology. It is to be further understood thatthe first, intermediate and second cathode films may also include pores38 (such as pores 38 in discrete areas 18′, 18″ and 18′″, respectively,as shown in FIG. 7), and/or other changes in morphology.

[0075] Further, it is to be understood that several different methodsmay be used to make the compositional gradients of embodiments of thepresent invention, including but not limited to sputter deposition,impregnation, dip coating or other means, and the like. Further methodsinclude, but are not limited to asymmetric screen printing and/orasymmetric tape casting, both generally with dopant or pore formerdelivery from one side, colloidal spray deposition, and the like.Substantially all deposition methods are contemplated as being withinthe purview of the present invention, provided that there is someasymmetry (i.e., two-sources or more with different compositions whereinthe sources will not provide a homogenous distribution on the substrate,e.g. one source biased to one end and the other source biased to theother end).

[0076] It is to be understood that the electrolyte 14 may be formed fromany suitable material. In an embodiment of the present invention,electrolyte 14 is at least one of oxygen ion conducting membranes,proton conductors, carbonate (CO₃ ²⁻) conductors, OH⁻ conductors, andmixtures thereof.

[0077] In an alternate embodiment, electrolyte 14 is at least one ofcubic fluorite structures, doped cubic fluorites, proton-exchangepolymers, proton-exchange ceramics, and mixtures thereof. In a furtheralternate embodiment, electrolyte 14 is at least one ofyttria-stabilized zirconia, samarium doped-ceria, gadoliniumdoped-ceria, La_(a)Sr_(b)Ga_(c)Mg_(d)O_(3−δ), and mixtures thereof.

[0078] It is to be understood that the anode 16 and cathode 18 may beformed from any suitable material, as desired and/or necessitated by aparticular end use. In an embodiment, each of the anode 16 and cathode18 is at least one of metals, ceramics and cermets.

[0079] In an embodiment of the present invention, some non-limitativeexamples of metals which may be suitable for the anode 16 include atleast one of nickel, platinum, palladium, and mixtures thereof. Somenon-limitative examples of ceramics which may be suitable for the anode16 include at least one of Ce_(x)Sm_(y)O_(2−δ), Ce_(x)Gd_(y)O_(2−δ),La_(x)Sr_(y)Cr_(Z)O_(3−δ), and mixtures thereof. Some non-limitativeexamples of cermets which may be suitable for the anode 16 include atleast one of Ni—YSZ, Cu—YSZ, Ni—SDC, Ni—GDC, Cu—SDC, Cu—GDC, andmixtures thereof.

[0080] In an embodiment of the present invention, some non-limitativeexamples of metals which may be suitable for the cathode 18 include atleast one of silver, platinum, ruthenium, rhodium, and mixtures thereof.Some non-limitative examples of ceramics which may be suitable for thecathode 18 include at least one of Sm_(x)Sr_(y)CoO_(3−δ),Ba_(x)La_(y)CoO_(3−δ), Gd_(x)Sr_(y)CoO_(3−δ), and mixtures thereof.

[0081] In any of the embodiments described herein, the gas to which fuelcell 10 is exposed includes reactants and/or oxidants and/or mixturesthereof. In an embodiment, the reactants are fuels, and the oxidants areone of oxygen, air, and mixtures thereof.

[0082] It is to be understood that any suitable fuel/reactant may beused with the fuel cell 10 of the present invention. In an embodiment,the fuel/reactant is selected from at least one of hydrogen, methane,ethane, propane, butane, pentane, methanol, ethanol, higher straightchain or mixed hydrocarbons, for example, natural gas or gasoline (lowsulfur hydrocarbons may be desirable, e.g. low sulfur gasoline, lowsulfur kerosene, low sulfur diesel), and mixtures thereof. In analternate embodiment, the fuel/reactant is selected from the groupconsisting of butane, propane, methane, pentane, and mixtures thereof.Suitable fuels may be chosen for their suitability for internal and/ordirect reformation, suitable vapor pressure within the operatingtemperature range of interest, and like parameters.

[0083] It is to be understood that the “expected compositions” of gasdescribed herein are non-limitative, and for illustrative purposes. Assuch, it is to be understood that the discrete areas 16′/18′, 16″/18″,16′″/18′″ and/or individual anodes/cathodes A, B, C should be optimizedfor whatever fuel is chosen, and its reaction and consequent byproductsalong the fuel flow path.

[0084] In an embodiment of the present invention, the fuel cell 10 is asingle chamber fuel cell (FIGS. 1, 1A and 1B). FIG. 2 is an example ofan anode fuel/air mixture gradient for a single chamber fuel cell. Inembodiments of single chamber fuel cells, the gas is a mixture ofreactants and oxidants.

[0085] It is to be understood that it is not necessary for goodperformance of the fuel cell 10 to have leak tight separation betweenair, fuel and exhaust in embodiments of the present invention relatingto single-chamber fuel cells. When mixing fuel, air and/or exhaust, itmay be desirable to keep the dimensions in the fuel cell stack below thecritical length required for propagation of a flame; e.g. forhydrocarbons, a flame generally needs to be at least about 1-3 mm insize to exist at room temperature. Optionally or additionally, it may bedesirable to adjust the air-fuel mixture so as to run with excess (abovethe upper flammability limit) fuel (for example, the upper flammabilitylimit for propane is 9.6%); and then to add more air when the oxygen isconsumed later in the stack. It may be desirable to add air at severallocations in the stack. Alternately to running with excess fuel, it maybe desirable to adjust the air-fuel mixture so as to run with excess(below the lower flammability limit) air (for example, the lowerflammability limit for propane is 2.2%); and then to add more fuel whenthe fuel is consumed later in the stack. It may be desirable to add fuelat several locations in the stack. It is believed apparent that amixture of multiple flammable gases will have a different flammabilitylimit than the flammability limit of the gases individually. Thus, iffor example, carbon monoxide (as a reaction product) is combined withpropane (as a fuel) later in the cell, the lower flammability limit ofthe mixture is 3.3%, while the upper limit is 10.9% according to LeChatelier's Principle.

[0086] In an alternate embodiment of the present invention, the fuelcell 10 is a dual chamber fuel cell (FIG. 1C). In embodiments of dualchamber fuel cells, the gas is one of reactants and oxidants. Oxidantsare carried to the cathode 18 of each of the fuel cell assemblies 12,and reactants are carried to the anode 16 of each of the fuel cellassemblies.

[0087] It is to be understood that the gas flow may be in any suitabledirection as desired and/or necessitated by a particular end use. Forexample, the gas flow direction may be a direction reverse of thatindicated by arrow A (FIG. 1), if desired. If such gas flow direction isreversed, it is to be further understood that inlet 20 and outlet 22would also be the reverse of those shown in the figures, and discreteareas 16′, 18′ and 16′″, 18′″ would be the reverse of those shown in thefigures.

[0088] It is to be understood that the anode 16 and/or cathode 18 are tobe optimized according to an expected composition of the gas to which it16, 18 is exposed. It is to be further understood that many embodimentsof the anodes 16/cathodes 18 are contemplated as being within thepurview of the present invention. For example, each of anode 16 and/orcathode 18 may include any of the appropriate structures and/orcompositions for discrete areas 16′/18′, 16″/18″ and 16′/18′″ (as wellas other appropriate structures/compositions), in any combinationthereof. As one non-limitative example, discrete area 16′″ of anode 16may be formed from Ni_(y):Ce_(1−x)Sm_(x)O₂, and also may include largepores 36 as shown in FIG. 5.

[0089] The gas phase and/or compositional and/or structural gradients ofthe anodes 16/cathodes 18 of embodiments of the present invention allowfor better fuel utilization, better thermal stability of the fuel cell10/stack 40, 40′, and enhanced performance.

[0090] While several embodiments of the invention have been described indetail, it will be apparent to those skilled in the art that thedisclosed embodiments may be modified. Therefore, the foregoingdescription is to be considered exemplary rather than limiting, and thetrue scope of the invention is that defined in the following claims.

What is claimed is:
 1. A fuel cell, comprising: a flow passage having agas stream flowing therethrough; and at least one electrode operativelydisposed in the flow passage, and having at least one discrete,catalytically active area having a composition and a structure, whereinat least one of the composition and structure is predetermined basedupon an expected composition of the gas stream to which the at least onediscrete area is exposed.
 2. The fuel cell as defined in claim 1,further comprising an inlet adjacent an entrance to the flow passage,wherein the at least one electrode has an inlet end region proximate theinlet, and wherein the at least one discrete area is located at theinlet end region.
 3. The fuel cell as defined in claim 2 wherein the atleast one electrode is an anode, wherein the expected composition of thegas stream comprises substantially unreformed hydrocarbon fuel, andwherein the at least one of the composition and structure of the atleast one discrete area is optimized therefor.
 4. The fuel cell asdefined in claim 2 wherein the at least one electrode is a cathode,wherein the expected composition of the gas stream comprises a gasstream substantially undepleted of oxidants, and wherein the at leastone of the composition and structure of the at least one discrete areais optimized therefor.
 5. The fuel cell as defined in claim 1, furthercomprising an outlet adjacent an exit from the flow passage, wherein theat least one electrode has an outlet end region proximate the outlet,and wherein the at least one discrete area is located at the outlet endregion.
 6. The fuel cell as defined in claim 5 wherein the at least oneelectrode is an anode, wherein the expected composition of the gasstream comprises at least one of substantially reformed or partiallyreformed hydrocarbon fuel, byproducts thereof, and mixtures thereof, andwherein the at least one of the composition and structure of the atleast one discrete area is optimized therefor.
 7. The fuel cell asdefined in claim 5 wherein the at least one electrode is a cathode,wherein the expected composition of the gas stream comprises a gasstream substantially depleted of oxidants, and wherein the at least oneof the composition and structure of the at least one discrete area isoptimized therefor.
 8. The fuel cell as defined in claim 1 wherein theflow passage has a midpoint, wherein the at least one electrode has amidpoint region proximate the midpoint, and wherein the at least onediscrete area is located at the midpoint region.
 9. The fuel cell asdefined in claim 8 wherein the at least one electrode is an anode,wherein the expected composition of the gas stream comprises at leastone of substantially unreformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof, and wherein the at least oneof the composition and structure of the at least one discrete area isoptimized therefor.
 10. The fuel cell as defined in claim 8 wherein theat least one electrode is a cathode, wherein the expected composition ofthe gas stream comprises a gas stream partially depleted of oxidants,and wherein the at least one of the composition and structure of the atleast one discrete area is optimized therefor.
 11. A fuel cell,comprising: a flow passage having a gas stream flowing therethrough; andan inlet at one end of the flow passage; at least one electrodeoperatively disposed in the flow passage, and having at least onediscrete, catalytically active area having a composition and astructure, wherein at least one of the composition and structure ispredetermined based upon an expected composition of the gas stream towhich the at least one discrete area is exposed; and a manifold,operatively and fluidly connected to the flow passage, for adding atleast one of reactants and oxidants in at least one area downstream fromthe inlet.
 12. An electronic device, comprising: a load; and the fuelcell of claim 1 connected to the load.
 13. A fuel cell, comprising: aflow passage having a gas stream flowing therethrough, the flow passagehaving a midpoint; an inlet adjacent an entrance to the flow passage; anoutlet adjacent an exit from the flow passage; and at least oneelectrode operatively disposed in the flow passage, and having a first,second and third discrete, catalytically active area, each area having acomposition and a structure, wherein at least one of the composition andstructure is predetermined based upon a first, second and third expectedcomposition of the gas stream to which each of the first, second andthird discrete areas is exposed, respectively; wherein the at least oneelectrode has an inlet end region proximate the inlet, an outlet endregion proximate the outlet, and a midpoint region proximate themidpoint, and wherein the first discrete area is located at the inletend region, the second discrete area is located at the midpoint region,and the third discrete area is located at the outlet end region.
 14. Thefuel cell as defined in claim 13 wherein the at least one electrode isan anode, wherein the first expected composition of the gas streamcomprises substantially unreformed hydrocarbon fuel, and wherein the atleast one of the composition and structure of the first discrete area isoptimized therefor; wherein the second expected composition of the gasstream comprises at least one of substantially unreformed or partiallyreformed hydrocarbon fuel, byproducts thereof, and mixtures thereof, andwherein the at least one of the composition and structure of the seconddiscrete area is optimized therefor; and wherein the third expectedcomposition of the gas stream comprises at least one of substantiallyreformed or partially reformed hydrocarbon fuel, byproducts thereof, andmixtures thereof, and wherein the at least one of the composition andstructure of the third discrete area is optimized therefor.
 15. The fuelcell as defined in claim 14 wherein the composition of the firstdiscrete area has as a main component thereof. LaCr(Ni)O₃, wherein thecomposition of the second discrete area has as a main component thereof.La(Sr)CrO₃, and wherein the composition of the third discrete area hasas a main component thereof. La(Sr)Cr(Mn)O₃.
 16. The fuel cell asdefined in claim 14 wherein the composition of the third discrete areaincludes a first amount of nickel, wherein the composition of the seconddiscrete area includes a second amount of nickel, which is less than thefirst amount of nickel, and wherein the composition of the firstdiscrete area includes a third amount of nickel, which is less than thesecond amount of nickel.
 17. The fuel cell as defined in claim 14wherein the structure of the third discrete area comprises pores,wherein the structure of the second discrete area comprises poressmaller than the third discrete area pores, and wherein the structure ofthe first discrete area comprises pores smaller than the second discretearea pores.
 18. The fuel cell as defined in claim 14 wherein thestructure of at least one of the first, second and third discrete areascomprises a varied pore size distribution.
 19. The fuel cell as definedin claim 18 wherein the varied pore size distribution includes a dualdistribution comprising a combination of large, transport pores andnanopores.
 20. The fuel cell as defined in claim 13 wherein the at leastone electrode is a cathode, wherein the first expected composition ofthe gas stream comprises a gas stream substantially undepleted ofoxidants, and wherein the at least one of the composition and structureof the first discrete area is optimized therefor; wherein the secondexpected composition of the gas stream comprises a gas stream partiallydepleted of oxidants, and wherein the at least one of the compositionand structure of the second discrete area is optimized therefor; andwherein the third expected composition of the gas stream comprises a gasstream substantially depleted of oxidants, and wherein the at least oneof the composition and structure of the third discrete area is optimizedtherefor.
 21. The fuel cell as defined in claim 20 wherein the cathodeis formed from a main material, and wherein the composition of the thirddiscrete area includes a first amount of material more catalyticallyactive than the main material, wherein the composition of the seconddiscrete area includes a second amount of more catalytically activematerial, which is less than the first amount of more catalyticallyactive material, and further includes a first amount of material lesscatalytically active than the main material, and wherein the compositionof the first discrete area includes a second amount of lesscatalytically active material, which is more than the first amount ofless catalytically active material.
 22. The fuel cell as defined inclaim 20 wherein the structure of the third discrete area comprisespores, wherein the structure of the second discrete area comprises poressmaller than the third discrete area pores, and wherein the structure ofthe first discrete area comprises pores smaller than the second discretearea pores.
 23. The fuel cell as defined in claim 20 wherein thestructure of at least one of the first, second and third discrete areascomprises a varied pore size distribution.
 24. The fuel cell as definedin claim 23 wherein the varied pore size distribution includes a dualdistribution comprising a combination of large, transport pores andnanopores.
 25. The fuel cell as defined in claim 13, further comprisinga manifold, operatively and fluidly connected to the flow passage, foradding at least one of reactants and oxidants in at least one areadownstream from the inlet.
 26. A fuel cell stack, comprising: an inlet;an outlet; a flow passage disposed between the inlet and the outlet, andhaving a gas stream flowing therethrough; and a plurality of electrodeseach having a structure and a composition, and operatively positionedwithin the flow passage from proximate the inlet to proximate the outletand positions therebetween, wherein at least one of the structure andcomposition of each of the plurality of electrodes is predeterminedbased upon an expected composition of the gas stream at an area of thefuel cell stack in which the electrode is positioned.
 27. The fuel cellstack as defined in claim 26 wherein each of the plurality of electrodesis an anode.
 28. The fuel cell stack as defined in claim 26 wherein eachof the plurality of electrodes is a cathode.
 29. A fuel cell,comprising: at least one electrode operatively disposed in the fuelcell, and having a catalytically active surface; and means formaintaining a substantially uniform maximum catalytic activity over thesurface of the at least one electrode.
 30. The fuel cell as defined inclaim 29 wherein the at least one electrode is an anode.
 31. The fuelcell as defined in claim 29 wherein the at least one electrode is acathode.
 32. A fuel cell, comprising: a flow passage having an inlet atone end; at least one electrode operatively disposed in the flowpassage, and having a catalytically active surface; means formaintaining a substantially uniform maximum catalytic activity over thesurface of the at least one electrode; and means, operatively andfluidly connected to the flow passage, for adding at least one ofreactants and oxidants in at least one area downstream from the inlet.33. A method for making a fuel cell anode, comprising the steps of:depositing a first film on a first end region of a substrate, whereinthe first film is preferentially catalytically active towardsubstantially unreformed hydrocarbon fuel; and depositing a second filmon a second end region of the substrate, the second end region opposedto the first end region, wherein the second film is preferentiallycatalytically active toward at least one of substantially reformed orpartially reformed hydrocarbon fuel, byproducts thereof, and mixturesthereof.
 34. The method as defined in claim 33, further comprising thestep of depositing an intermediate film on a region of the substrateintermediate the first end region and the second end region, wherein theintermediate film is preferentially catalytically active toward at leastone of substantially unreformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof.
 35. The method as defined inclaim 34 wherein each of the first, intermediate, and second films hasas a main component thereof a nickel-samaria doped ceria cermet, andwherein the method further comprises the step of biasing inclusion ofnickel toward the second film.
 36. The method as defined in claim 34wherein the first film has as a main component thereof. LaCr(Ni)O₃,wherein the intermediate film has as a main component thereof.La(Sr)CrO₃, and wherein the second film has as a main component thereof.La(Sr)Cr(Mn)O₃.
 37. The method as defined in claim 34 wherein the secondfilm comprises pores, wherein the intermediate film comprises poressmaller than the second film pores, and wherein the first film comprisespores smaller than the intermediate film pores.
 38. A fuel cell anodeformed by the process of claim
 33. 39. A method for making a fuel cellcathode, comprising the steps of: depositing a first film on a first endregion of a substrate, wherein the first film is preferentiallycatalytically active toward a gas stream substantially undepleted ofoxidants; and depositing a second film on a second end region of thesubstrate, the second end region opposed to the first end region,wherein the second film is preferentially catalytically active toward agas stream substantially depleted of oxidants.
 40. The method as definedin claim 39, further comprising the step of depositing an intermediatefilm on a region of the substrate intermediate the first end region andthe second end region, wherein the intermediate film is preferentiallycatalytically active toward a gas stream partially depleted of oxidants.41. The method as defined in claim 40 wherein each of the intermediateand second films has an amount of a material more catalytically activethan the substrate, and wherein the method further comprises the step ofbiasing inclusion of the more catalytically active material toward thesecond film.
 42. The method as defined in claim 40 wherein each of thefirst and intermediate films has an amount of a material lesscatalytically active than the substrate, and wherein the method furthercomprises the step of biasing inclusion of the less catalytically activematerial toward the first film.
 43. The method as defined in claim 40wherein the second film comprises pores, wherein the intermediate filmcomprises pores smaller than the second film pores, and wherein thefirst film comprises pores smaller than the intermediate film pores. 44.A fuel cell cathode formed by the process of claim
 39. 45. A method ofusing a fuel cell, comprising the step of: operatively connecting thefuel cell to at least one of an electrical load and an electricalstorage device, the fuel cell comprising: a flow passage having a gasstream flowing therethrough; and at least one electrode operativelydisposed in the flow passage, and having at least one discrete,catalytically active area having a composition and a structure, whereinat least one of the composition and structure is predetermined basedupon an expected composition of the gas stream to which the at least onediscrete area is exposed.
 46. A fuel cell, comprising: a flow passagehaving an inlet at one end; at least one electrode operatively disposedin the flow passage, and having a catalytically active surface; andmeans, operatively and fluidly connected to the flow passage, for addingat least one of reactants and oxidants in at least one area downstreamfrom the inlet, wherein the adding means substantially maintains auniform maximum catalytic activity over the surface of the at least oneelectrode.